Polyarylene sulfides are resins having properties suitable for engineering plastics such as excellent heat resistance, barrier properties, chemical resistance, electrical insulating properties, moist heat resistance, and flame resistance. In particular, polyphenylene sulfide resins can be molded by injection molding and extrusion molding into various molded parts such as films, sheets, and fibers and have been widely used in the fields of electrical and electronic components, machine parts, automotive parts, and other, parts requiring heat resistance and chemical resistance.
Polyphenylene sulfide resins, however, are inferior to other engineering plastics in shock resistance, toughness, flexibility, and moldability and, to improve these properties, attempts have been made at modification by combination with dissimilar polymers. For example, one approach to providing polyphenylene sulfide with toughness and flexibility is to blend an olefinic elastomer. In this case, however, heat aging proceeds significantly under high-temperature conditions because of low heat resistance of the olefinic elastomer, resulting in low toughness and flexibility.
Against this background, studies have been made on methods of combining with polydimethylsiloxane, serving as a high-toughness and high-flexibility component that can withstand the use under high-temperature conditions.
Furthermore, since there is a limit to the modification by polymer alloying, studies have been made to provide higher toughness and higher flexibility, on combination methods for drastically modifying the main chain skeleton of polyphenylene sulfide by chemically bonding the polyphenylene sulfide to polydimethylsiloxane into a copolymer.
For example, studies have been made on methods of polymer alloying by blending polyphenylene sulfide with polydimethylsiloxane (see, for example, JP 2004-300270 A).
Furthermore, there have been reported methods of polymer alloying by blending a poly(ether imide-siloxane) copolymer, serving as a high-toughness and high-flexibility component to increase the compatibility between polyphenylene sulfide and polydimethylsiloxane (see, for example, JP 2012-46721 A).
As a method of producing a copolymer of polyphenylene sulfide and polydimethylsiloxane, there has been reported, for example, a method including introducing functional groups into the main chain of polyphenylene sulfide and reacting the polyphenylene sulfide with polydimethylsiloxane (see, for example, JP 07-179610 A).
Alternatively, there has been reported a method including block copolymerizing a polyphenylene sulfide with functional groups introduced into the end of the main chain with polydimethylsiloxane (see, for example, JP 64-45433 A).
According to the method of JP '270, the two polymers are incompatible with each other and not finely dispersed and, therefore, satisfactorily improved toughness and flexibility cannot be achieved.
According to the method of JP '721, provided is merely a blend of polyphenylene sulfide and a flexible component, and it was difficult to provide dramatically improved toughness and flexibility.
According to the method of JP '610, copolymerization reaction starts from the functional groups in the main chain of polyphenylene sulfide, and thus a copolymer in which polydimethylsiloxane is grafted is prepared. This copolymer is, therefore, not a block copolymer in which polydimethylsiloxane, a soft segment, is introduced into the main chain, and this polymer structure is not optimal to provide high flexibility and does not produce a sufficient modification effect.
According to the method of JP '433, the polyphenylene sulfide subjected to block co-polymerization has a low molecular weight comparable to those of oligomers and also has a small functional group content. Thus, block copolymerization reaction does not proceed sufficiently, only to provide a block copolymer having a low molecular weight. Furthermore, since the chain length of polyphenylene sulfide segments in the block copolymer is short, heat resistance and chemical resistance, which are inherent in polyphenylene sulfide, may be impaired.
As described above, it has been difficult, in the related art, to simultaneously achieve heat resistance, chemical resistance, high toughness, high flexibility, and heat aging resistance in polyphenylene sulfide.
It could therefore be helpful to provide a novel polyphenylene sulfide block copolymer having high flexibility and toughness with no reduction in heat resistance and chemical resistance, which are inherent in polyphenylene sulfide.